This invention relates to the removal of acidic constituents such as carbon dioxide, hydrogen sulfide, and other sulfur compounds from natural gas. "Sweetening", as it is termed, is normally accomplished by an absorption process, with or without chemical reaction, using solutions of chemical or physical reactants, or combinations of both types of reactants. Most known processes are regenerative in nature and are based on the absorption-stripping principle.
More specifically, natural gas treatment or "sweetening" is generally accomplished by contacting the sour natural gas stream which may contain varying quantities of carbon dioxide, hydrogen sulfide and other sulfur compounds, with any one of a variety of known physical or chemical solvent reagents, or a combination of both, in an absorber. The solvent reacts chemically and/or absorbs acid fractions in the sour natural gas producing a marketable natural gas stream. The solvent is generally regenerated by flashing the acid gas therefrom by pressure reduction and/or heating the solvent to its boiling point in a stripper column. Solvent recovered from the regeneration system is cooled and recirculated back to the absorber column.
It is to be noted that absorption of acid molecules by either physical reactants or chemical solvents is exothermic in nature and, accordingly, the system temperature rises. As the system temperature increases the reaction rate slows, approaching an equilibrium condition. At a given temperature for each system the reaction stops and regeneration starts. Moreover, when the sweetening reaction slows, unreacted acid compounds may contaminate the system.
Another recognized problem is that the denuding of acid gas from the solvent in the solvent regeneration stripper is seldom complete and the residual acid components sometimes tie up as much as 20% of the reactive solvent, seriously reducing its effectiveness and/or requiring higher solvent circulation rates to accomplish the desired result. Thus, energy consumption for operating a gas sweetening unit is generally proportional to the solvent circulation rate.
In natural gas systems containing both carbon dioxide and hydrogen sulfide it is generally important that the amount of unreacted hydrogen sulfide be limited to a maximum of 4 ppm (parts per million) in the treated stream while the allowable carbon dioxide content may commonly reach 2,000 to 30,000 ppm in the system. Therefore, in order to denude the solvent to a point where essentially all of the hydrogen sulfide will be absorbed, excessive amounts of carbon dioxide must be first reacted and then regenerated to insure that essentially all of the hydrogen sulfide will be removed from the treated stream.
In most exothermic reactions an increase in temperature is detrimental to the reaction. Elevations in temperature of up to 30.degree. to 40.degree. F. are common in the absorber particularly in systems containing significant quantities of sour components. Such temperature increases in the sour system increase the corrosion rate. Also, the amount of energy required to regenerate the reagent rises significantly as the degree of regeneration increases. To overcome the potential negative effect of the higher system temperature excessive quantities of reagent are commonly circulated.